One type of offshore system includes a turret that lies in a moonpool of a vessel hull, or outboard of the hull, and a bearing structure that allows the vessel to weathervane (rotate without limit about a vertical axis) around the turret. The turret is anchored to the seafloor and fluid lines usually extend from wells or pipelines at the seafloor up to the turret. The bearing structure includes an upper bearing assembly and sometimes a lower bearing assembly. The upper bearing assembly supports the weight of the turret and the weight of the mooring structure and hoses attached thereto, which may amount to thousands of tons.
The upper bearing assembly has previously been a roller bearing, which has low friction so the turret will turn only a few degrees before the rollers roll. However, there are serious disadvantages in the use of roller bearings. One disadvantage is that reliable roller bearings require raceways that are forged before machining, to provide strength to resist the concentrated forces of individual rollers. Presently, there is no forging equipment available in the world that can forge raceways of greater than eight meters diameter. Some large turrets have diameters of up to twenty meters. Although the raceways can be formed of forged segments that are welded together, this results in reduced strength and in irregularities in the raceway surfaces under load. In addition, the cost for large roller bearings is very high. An upper bearing structure for supporting the weight of a turret on a vessel hull, which avoided the above disadvantages, especially for large turrets of over eight meters diameter, would be of value.